if the POA provides for an alternate or successor Agent, then it should just be a matter of your sending a letter to the successor/alternate. I would also send it to the principal and anyone else who you have been dealing with as agent. You may want to have the letter notarized, as well.
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You could execute an affidavit declining to serve as the agent under the power of attorney. If there is a back up agent, that person would then step in. I would also send a letter and the affidavit to her son and sisters.
It is a good idea for your own protection to consult with an elder law attorney in Arizona to make sure you are not left vulnerable or liable in any way.
Powers of attorney frequently fail if the appointed attorney-in-fact can not or does not want to serve and the document does not list a successor who is able and willing to serve. That is one reason that people should consult with an experienced elder law attorney when preparing powers of attorney. You can refuse to serve (follow the advice that has been given) and then your friends family may need to step up and petition for guardianship. You only mentioned a durable general power of attorney. Does it contain a health care power? Without that authority, you can't make health decisions for her anyway.